When carbides, nitrides or carbonitrides are prepared from metals or metalloids, the educts are reacted in homogeneous distribution in order to give rise to finely divided, high quality products.
In some carbothermal processes, homogeneous distribution is achieved by starting with mixtures of the corresponding oxides and organic hydrocarbon compounds (in most cases as liquids) which have a high residue of carbon when decomposed by heat and are therefore available as a finely divided source of carbon for the reduction of metal oxides and oxides of non-metals.
An additional improvement is obtained if the oxidic compound is also used in an extremely finely divided form. Thus according to Ceramic Bulletin Vol. 63, No. 8 (1984), a finely divided or liquid metallic or non-metallic component is put into the process with colloidal SiO.sub.2 or methyl trimethoxy silane. The colloidal SiO.sub.2 is, however, prepared by an expensive and complicated method of flame hydrolysis and the alcoholate is prepared by a method of alcoholysis, which render this process uneconomical. Another disadvantage is that in addition to an organic hydrocarbon compound, water is used as solvent or gelling agent, which is withdrawn from the gel by a lengthy process of freeze drying at -50.degree. C.
The gels are subsequently converted into SiO.sub.2 and C at temperatures from 500.degree. to 800.degree. C. and reacted for 4 to 16 hours at a temperature of 1600.degree. C. to form SiC. The products obtained are still contaminated with unreacted SiO.sub.2 in spite of this elaborate process.
According to U.S. Pat. No. 3,085,863, SiCl.sub.4 is used as starting material instead of the alkoxide or colloidal SiO.sub.2 for the preparation of the SiC powder. Owing to the aqueous sugar solution used as source of carbon, however, a time consuming process of distillation (24 hours at 200.degree. to 300.degree. C.) must subsequently be carried out for dehydrating the silica gel before the conversion to SiC can be carried out at 1800.degree. C. Since the hydrolysis reaction is in any case vigorous, the introduction of SiCl.sub.4 must be carried out extremely slowly. The process is further complicated by the fact that blockages are liable to occur in the part of the apparatus where the SiCl.sub.4 is introduced.
According to Advanced Ceramic Materials 2 (3A) (1987) 253-56, alkoxides are again used for the preparation of Si.sub.3 N.sub.4 and AlN but these alkoxides are precipitated as gels on lamp black by a process of hydrolysis. Apart from the cost intensive starting materials used, the dehydration at low pressure which the process requires make it questionable whether the process can be carried out economically on an industrial scale.
Moreover, the powders prepared at 1500.degree. C. have unacceptably high oxygen contents, amounting to 2% O for Si.sub.3 N.sub.4 and 3.multidot.1% O for AlN.
A process for the preparation of boron nitride and boron carbide by pyrolysis of a polymeric ester of boric acid and glycerol is described in Chemistry Letters (1985) 691-692. The disadvantage of the boron-containing ceramic powders prepared from these organoborate esters is the poor reaction to the end product. The boron nitride obtained is heavily contaminated with boron carbide.
Patent Application EP-A No. 0 239 301 also describes a process for the preparation of nitrides and carbides by the thermal decomposition (up to 1800.degree. C.) of esters of polyhydric alcohols.
In contrast to the process described above, the starting materials used for esterification are not oxides but, as mentioned in all the Patent examples, the cost intensive metal alkoxides or alkoxides of non-metals.
The alcohol component used is a compound containing at least two or preferably more than two hydroxyl groups, by means of which the cross-linking to a polymer takes place.
The polyhydric alcohols, ethylene glycol and glycerol, undergo esterification with the alkoxides of metals or non-metals with liberation of alcohol. This alcohol must be distilled off by a lengthy process in order to shift the equilibrium to the side of the product.
A small quantity of an organic compound, e.g. furfuryl alcohol, is optionally added to increase the carbon content.
The process is therefore uneconomical on account of the expensive starting materials (alcoholates) and the costly and time consuming steps of the process.
It is an object of the present invention to provide a process for the preparation of nitrides, carbides and carbonitrides in which inexpensive and highly pure chemicals may be used as starting materials and which does not have the disadvantages of the processes described above. Furthermore, the starting materials should be capable of being converted into the carbides, nitrides or carbonitrides without the costly and time consuming process steps described above.
A process which fulfills these requirements has now surprisingly been found. In this process, inexpensive halides may be directly used as starting materials without the elaborate preliminary step of esterification or hydrolysis.